I have recently started using a magnetic stirer to make my starter. The two 5gal batches that I have made since started blowing off the air lock, and/or the blow off tube and making a mess.
I am using 7.5gal plastic bucket with about 8" of head space, but during the first 6 hrs it foams all the space and out the air lock. I tried a hose in the stopper to a 1gal catch bucket, but it blows off after making a bubble bath over and out of the catch bucket. What is my problem or solution?

What you described is a great thing, not a bad one. You have increased the overall viability and density of your yeast pitching. This will lead to a more vigorous fermentation than you may be used to. 8" of head space isn't a bunch, so I'm not surprised you are foaming it in. You can either 1) reduce the volume or 2) go to carboy fermentation. The reason I say carboy fermentation is that in a 7.5 ga. carboy, you can fit a 5/8" blowoff hose through the neck. This will go a long way towards reducing the mess because there is less pressure due to the increased diameter of the hose. Once the krausen stops blowing off, you simply replace the hose with a stopper and airlock. None the less... don't stop what you are doing with your starter. Any loss of beer is offset by the enhanced quality the higher pitching rate will give.

In my yeast business, I propagate yeast from slants to pitchable brewing quantities, usually for 600 or so gallons (20 BBL). During the stage where I increase the overall cell count, I use a magnetic stirrer and a stainless steel diffuser that delivers oxygen continuously. Except for the O2, you are doing the same thing as me. If you even want better results, try batch feeding it with lower gravity wort. Start with 50 ML of 1.020 wort and double the quantity every 24 hours. The reason this works better is because you are keeping the yeast in more of a reproductive phase than a fermentation phase. This is why you don't want the wort at a higher gravity... it is so they have little to ferment. With each subsequent addition, they go back towards reproduction and mass increases. With the addition of oxgen, they remain almost exclusively in the reproductive phase since an anaerobic environment is what kicks them fully into fermentation mode.

Don't confuse this with normal starter production... in those, you need to pitch the yeast into a full volume starter (usually 1,000 ~ 2,000 ml.) with a gravity similar to the beer that you are brewing. This is because the yeast will be in the fermentation stage when you pitch it and if there is little wort difference, there will be less delay.